The damage amount is calculated for each spell tick at the time of the tick (every 4s). Thus, applying debuffs like Vulnerability Hex and Affliction Hex after Inferno was cast will increase damage for subsequent spell ticks.

Inferno can be cast on creatures or locations not in line of sight. A mage can cast this spell on creatures in a different room when using the overhead view. This allows the mage to stay out of physical harm by staying out of reach of those creatures.